1. Field of the Invention
The invention concerns a link adaptation process.
2. Discussion of the Related Art
In known wireless communication systems, information are transmitted via the air interface between two entities of the systems via the physical layer following the ISO-OSI model.
Depending on the current radio link conditions, several physical layer modes can be selected. A physical layer mode specifies a coding rate and modulation conditions. A link adaptation process is implemented for selecting the physical layer modes (PHY modes).
For example, such a link adaptation process is necessary in a High PErformance Radio Local Area Network type 2 (HIPERLAN/2) standard. In the following, the HIPERLAN/2 standard will be used often as an example of framework to describe the context of the invention. However, the field of application of this invention goes beyond the boundaries of the HIPERLAN/2 standard and can be considered for any wireless communication system implenting a link adaptation process.
The Hiperlan/2 standard defines PHYsical (PHY) and Data Link Control (DLC) layers. The link adaptation process is part of the DLC layer.
The air interface is based on dynamic Time-Division Multiple Access (TDMA) with Time-Division Duplex (TDD). Orthogonal Frequency Division Multiplexing (OFDM) has been selected as modulation scheme for H/2 due to its good performance on frequency-selective fading channels. BPSK (Binary Phase Shift Keying), QPSK (Quaternary Phase Shift Keying), 16QAM (16 points Quadrature Amplitude Modulation) and 64QAM (64 points Quadrature Amplitude Modulation) (optional) are the supported sub-carrier modulation schemes.
The link adaptation process selects the PHY modes by implementing a criteria dependent on the radio link conditions.
In the known link adaptation processes, the criteria implemented for switching between the PHY modes considers the Signal-to-Noise Ratio (SNR) or Signal to Noise and Interference Ratio (SINR) as an input.
The PHY modes are selected to obtain an acceptable Bit Error Rate (BER) or Packet Error Rate (PER) which is necessary for the communication depending on the type of data which are transmitted.
The criteria is adapted to select a PHY mode which is able to obtain a requested BER performance considering the current measured SNR or SINR.
In fact, the measured SINR is an averaged SINR since the channel is a time-varying channel. Besides, the relation between the SINR and the BER is not straightforward because it is also very dependent on the transmission channel.
A way to implement the SINR criteria approach is to consider the typical “worst-case” scenario for the channel.
Namely, the prediction of the BER according to the SINR measured will lead to the averaged “worst-case” propagation channel to be used as reference. The thresholds used to select the PHY modes are determined accordingly.
However, information on the SINR combined with information on the channel profile would enable to determine more accurately the BER performance. Indeed, the BER and PER performance does not only depend on SINR but also on the characteristics of the frequency selective fading channel.
A solution to this problem is proposed in “Misunderstandings about link adaptation for frequency selective fading channels”, M. Lampe et al., PIMRC conference, September 2002. This document suggests a link adaptation process based on the SINR with statistics on the type of fading channel.
The problem mentioned in this document is how to predict the PER or BER taking into account SINR and channel statistics.
The type of channel statistics mentioned explicitly is the estimate variance of the absolute value of the channel response in the frequency domain:
      I          v      ⁢                          ⁢      ar        =            1      N        ⁢                  ∑        i        N            ⁢                          ⁢                        (                                                                  ρ                i                                                    -                                                        ρ                _                                                            )                2            where ρi is the channel coefficient (fading channel response in the frequency domain) of the ith sub-carrier of the OFDM signal.
However, the results presented in this article on FIG. 10 are only for the particular channel A of Hiperlan/2 standard and therefore it is difficult to understand how this criteria can be effectively used to become independent from knowledge a-priori of the fading channel.